Meanwhile, in your other example, Elvis has a good deal of primary influence though (rockabilly, power ballads, electric bass solos - I'm looking, but haven't found an earlier example than his in rock, if I do, I'd change that), and obviously in terms of cultural influence he's off the charts. So yeah, primary influence is the single biggest achievement you can get for anything, but there's not always much of it to go around and so the secondary influence is what ultimately will wind up affecting the rankings the most overall and as you state, the more popular artists have the built-in ability to achieve that more than most. As long as nobody discounts the greater credit for primary influence and the original innovation.
See, where I disagree with you is this: if we do find some obscure record from 1955 with a bass solo, I don't believe that that artist ought to get as much credit, or more, for that than Elvis. Because it's much more likely that people are doing electric bass solos because Elvis did one than because that other guy did one. That guy *might* deserve substantial credit *if* Elvis got the idea from him. Or maybe not.
In the case of rockabilly, I don't think there's any definition of rockabilly you could come up with where Elvis' first Sun record was the first record that qualifies. But Elvis didn't need to do it first to be the one who defined the genre, and therefore to be the primary person who gets credit for that influence.
I have no idea how you're defining "power ballads" here.
OH... and one last good thing about this debate is with people suggesting it's impossible to have every recording and so forth, isn't that the goal of all this? These lists, this site, these conversations? To discover as much music as humanly possible, to hear it all and understand how it evolved and how it all ties together? I think of all the four criteria we have, the way influence is credited offers the most opportunity to actually discover things you otherwise wouldn't hear or even look into.
Hmmm...I don't know, I was talking about what's possible, not what's desirable. If I had the means to spend huge amounts of time listening to music, would I try to listen to everything? I don't know.